The invention concerns a stent having a metallic, relatively radiolucent carrier structure and at least one marker element comprising comparatively radiopaque material.
Stents are endovascular prostheses which serve inter alia for the treatment of stenoses, that is to say, vessel constrictions. Stents usually have a tubular carrier structure which is open at both longitudinal ends of the tube and which is formed by legs and openings enclosed by the legs. Stents of that kind can usually assume two conditions, more specifically a compressed condition of a small diameter and an expanded condition of a comparatively larger diameter. In the compressed condition, such a stent can be introduced by means of a catheter into for example a blood vessel and can be positioned at a location to be treated. The stent is expanded or is allowed to expand of its own accord, at the treatment location. Stents which are not self-expanding are usually expanded by means of an inflatable balloon at a distal end of a catheter for insertion of the stent. Stents of that kind are therefore referred to as balloon-expanded. Other stents have the property of expanding of their own accord, for example by virtue of inherent spring forces. Stents of that kind are referred to as self-expanding. The self-expanding stents include in particular those which have a carrier structure comprising a shape memory metal such as nitinol, a known titanium nickel compound. Shape memory metals of that kind have the property of retaining a first shape or being plastically deformable below a given change temperature, and assuming a second shape when the change temperature is exceeded. In regard to stents, shape memory metals are used in such a way that the first shape corresponds to the compressed condition of a stent and the second shape corresponds to the expanded condition of a stent.
In the expanded condition of a stent, it serves, for example, for the treatment of vessel constrictions (stenoses), acting as a vessel support which keeps a blood vessel to be treated open at a constricted location. The expanded stent acts in opposition to the vessel constriction and supports the vessel wall in the region of the vessel constriction. For that purpose, the stent must enjoy adequate radial strength or carrying force. The carrier structure of the stent must also afford adequate surface coverage in order adequately to support the vessel constriction. On the other hand, the requirement for being able to expand the stent means that openings are necessarily present between the legs of the carrier structure of a stent. In the compressed condition of the stent, those openings can be substantially closed. In the expanded condition of the stent, the opening is enlarged in any case.
Besides an adequate carrying force, stents must also involve adequate flexibility with respect to their longitudinal axis in order to be able to follow movements of the vessel. In addition there is a wish for longitudinal changes in a stent upon expansion to be kept as small as possible. Finally the material of the stent or at least the surface thereof should be as body-compatible as possible.
Those requirements have resulted in various, very sophisticated and diversified stent configurations. The stents which are of interest here have a carrier structure produced from a metal tube as the starting material. Cutting the metal tube for example by means of a laser or spark erosion produces the legs, as remaining material. Cut legs of that kind have the great advantage over stents of the first generation, which were shaped from wire, that the geometry of the carrier structure can be optimized in regard to the various different demands involved.
That large number of demands made on stents further includes the requirement that the stent is to be capable of being positioned with the utmost accuracy. In general, the operation of positioning a stent is effected by means of imaging processes which, for example, operate with X-rays. In this connection, there is generally the disadvantage that materials which are suitable for the carrier structure of a stent frequently can only be detected with difficulty by means of the imaging processes used as the material forming the carrier structure is relatively radiolucent. It is therefore known to provide stents with what are referred to as X-ray markers containing a relatively radiopaque material which is easy to locate by means of the above-specified imaging processes. A known radiopaque material is for example, gold.
Usually, the need to provide X-ray markers in stents of the above-described kind forces compromises in terms of the stent design, which are possibly detrimental to others of the above-mentioned desired properties.
An aspect of the present invention is to provide a stent which is X-ray visible and which moreover combines together as many as possible of the desirable properties.